Howdy,
I absolutely love the idea of this project. Not only is it a brilliant exploit, it's also very expandable.
The Teensy 2.0 (even the Teensy++ 2.0) is a nice little development system and a great alternative to Arduino's hardware, but the ATMega32U4 is limited to a mere 16MHz, 32kB FLASH, and 2.5kB RAM, and a few peripherals (which should suffice, actually). There are a number of other microcontrollers out there that could increase the amount of awesome that could be accomplished.
For example, the AVR32 series from ATMEL has a lot more oomph behind it. The AT32UC3A0128 part features up to 91 MIPS at 66MHz, 12Mbps USB support, 10/100 ethernet, an EBI interface for external SRAM/SDRAM (128Mb+ external - though GCC is limited to 64kB due to a 16-bit address without hardware tricks; might be different for AVR32-GCC though). And, best of all about the AVR32? It's got enough resources for eLUA (Embedded LUA). Like Darren mentioned in the episode, you could include a SDHC card with plenty of storage, and simply dump LUA scripts onto the card which will get executed on startup. A couple DIP switches and you can select specific scripts for a needed application.
And, if you want to go completely overboard on performance, hacking a BeagleBoard into a slightly larger rubber duck could yield over 1,200 MIPS from its ARM Cortex-A8 600MHz processor, 128MB onboard RAM, 256MB NAND FLASH, SD Card, and other fun goodies for only $150. Oh, it's also got HDMI/DVI output which could be used as a small interface for the duck. The BeagleBoard can even run linux which could be used to execute Python scripts, etc. Oh the joy that could be had.
Cheers,
-robodude666